Contact lenses are widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness and far-sightedness (myopia and hypermetropia, respectively), astigmatism vision errors, and defects in near range vision usually associated with aging (presbyopia).
In a conventional molding process, a predetermined amount of a polymerizable or crosslinkable material (prepolymer) typically is introduced into a disposable mold comprising a female (concave) mold half and a male (convex) mold half. The female and male mold halves cooperate with each other to form a mold cavity having a desired geometry for a contact lens. Normally, a surplus of polymerizable or crosslinkable material is used so that when the male and female halves of the mold are closed, the excess amount of the material is expelled out into an overflow area adjacent to the mold cavity. The polymerizable or crosslinkable material remaining within the mold is polymerized or cross-linked by means of actinic radiation (e.g., UV irradiation, ionized radiation, microwave irradiation) or by means of heating. The starting material in the mold cavity is cured to form a lens while the excess material in the overflow area is partially or completely cured to form flash. After curing, the mold is separated into the male and female mold halves with the formed lens adhered onto either male or female mold half.
The molds that are used in the above-described process are created using tools or “master molds.” Two tools are needed to create one mold halve; hence four tools are needed to create an entire mold. Optical tools may be formed from any optically acceptable material stock, preferably in rod form. The stock is typically cut and roughed in to the general shape of the tool. The tool is then lathed to more exact specifications. The two tools that are used to create the portions of the mold halves that contact the contact lens prepolymers (front tools) may then be polished to remove any lathe marks. The polishing of the front tool without polishing the back tool causes the mold material to be affected by the different coefficient of friction on the top and bottom surfaces (boundary layer effects), which causes a non-uniform flow front. The present invention seeks to correct these inadequacies of the prior art